1. Field of the Invention
The present invention relates to an insulating substrate composed of insulative ceramic layers having a proper breakdown voltage, a method of manufacturing such an insulating substrate, and a semiconductor device employing the insulating substrate. The present invention also relates to a module semiconductor device such as a power semiconductor device having semiconductor chips to control large current.
2. Description of the Prior Art
Semiconductor chips used to control a small current of several milliamperes to several amperes. Presently, they are able to control a large current of several tens of amperes to about 100 amperes. There are module semiconductor devices that incorporate semiconductor chips in an insulative resin case to control a current of several hundreds of amperes to about 1000 amperes. The module semiconductor devices are widely used as power sources to drive vehicles or large motors in rolling plants and chemical plants.
The module semiconductor devices are capable of not only handing large current but also providing a high breakdown voltage of, for example, 5 kV. In the future, a breakdown voltage of 10 kV or higher will be required. A higher current value means higher heat generation. The module semiconductor devices must efficiently dissipate heat from semiconductor chips, and for this, they must be made of material of high thermal conductivity.
FIG. 1B is a sectional view showing a module semiconductor device 65 according to a prior art. Semiconductor chips 57 are joined to the top surface of an insulating substrate 51 with a solder layer 59. The bottom surface of the insulating substrate 51 is joined to the top surface of a base 60 with a solder layer 61. The base 60 is made of metal or a composite material of metal and ceramics. The semiconductor chips 57, solder layers 59 and 61, and insulating substrate 51 are sealed with insulative sealing resin 63 and are packed in an insulative resin case 64, to form the module semiconductor device 65. A water- or air-cooled heat sink 66 is fixed to the bottom surface of the base 60 with bolts 67.
FIG. 1A shows the insulating substrate 51 of the module semiconductor device 65 of FIG. 1B. The insulating substrate 51 consists of an insulative ceramic layer 52 and conductive layers 55 and 56. The conductive layers 55 and 56 are joined to the top and bottom surfaces of the ceramic layer 52, respectively, by direct bonding copper method or active metal brazing method.
The module semiconductor device 65 of the prior art dissipates heat from the semiconductor chips 57 to the insulating substrate 51, base 60, and heat sink 66. Therefore, the insulating substrate 51, in particular, the conductive layers 55 and 56 must have good thermal conductivity. For this, the conductive layers 55 and 56 are usually made of copper, aluminum, an alloy thereof, or a composite material thereof.
A breakdown voltage of the module semiconductor device 65 is determined by that of the semiconductor chips 57, which is determined by that of the insulating substrate 51. To improve the breakdown voltage of the module semiconductor device 65, it is necessary to improve the breakdown voltage of the insulating substrate 51. Improving the breakdown voltage of the insulating substrate 51 is achievable by thickening the ceramic layer 52. The ceramic layer 52 may be made of aluminum oxide (Al2O3) or aluminum nitride (AlN) having a good dielectric property.
A module semiconductor device has a layered structure of semiconductor chips and an insulative ceramic layer that have low thermal expansion coefficients, and conductive layers and a base that have high thermal expansion coefficients. When the semiconductor chips are energized, they generate heat to repeatedly apply large thermal stress onto these elements and sometimes crack the ceramic layer to cause a dielectric breakdown.
To cope with this problem, Japanese Unexamined Patent Publication No. 9-275166 forms a layer of refractory metal such as tungsten (W) and molybdenum (Mo) whose thermal expansion coefficients are close to that of an insulative ceramic layer of an insulating substrate, on each of the top and bottom surfaces of the ceramic layer, to relax thermal stress on the ceramic layer and reinforce the same. The refractory metal, however, has lower thermal conductivity than copper and aluminum, and therefore, is not always preferable in terms of cooling semiconductor chips. In addition, the conventional copper and aluminum plastically deform to relax thermal stress on an insulative ceramic layer. On the other hand, the refractory metal has a very high elastic coefficient and yield strength, and therefore, provides no stress relaxing effect. An analysis of thermal stress on refractory metal layers shows that high thermal stress occurs on the refractory metal layers. In addition, the fracture toughness of the refractory metal is not high. Due to these factors, the refractory metal layers have a high possibility of causing cracks due to thermal stress.
Japanese Unexamined Patent Publication Nos. 8-195450 and 8-195458 employ aluminum oxide to form an insulative ceramic layer to prevent cracks. Aluminum oxide may be stronger than aluminum nitride but has lower thermal conductivity than the aluminum nitride. This low thermal conductivity of aluminum oxide may further drop if reinforcing elements are added to aluminum oxide.
Materials used to form insulative ceramic layers generally have low fracture toughness and high crack sensitivity. Even a fine defect on the surface of an insulative ceramic layer may start a crack running across the thickness thereof. The inventors of the present invention studied the details of breaking behavior of insulating substrates through thermal cycles and found that the fracture toughness of insulative ceramic materials is very low compared with that of metal materials, and once a crack occurs on a layer made of an insulative ceramic material, it quickly propagates across the thickness of the layer. The insulative ceramic materials have a breakdown voltage of 10 kV or above per a thickness of 1-mm. However, even a fine crack across the 1-mm thickness deteriorates the breakdown voltage to that of air, i.e., about 3 to 4 kV. This may instantaneously cause a dielectric breakdown of a module semiconductor device that employs the ceramic layer. In high humidity, the breakdown voltage of air further deteriorates to cause a dielectric breakdown at a voltage lower than 3 kV or 4 kV.
If an insulative ceramic layer of 1-mm thick has no cracks running across the thickness thereof, it will maintain a breakdown voltage of 10 kV or higher. It is important, therefore, to prevent cracks on insulative ceramic layers.
Ceramic materials have individual strength values that widely vary from material to material. Accordingly, strength test data for a given ceramic material must statistically be processed with the use of standard deviations and Weibull distributions before determining a stress threshold for the ceramic material. Once the stress threshold is determined, it is used to design a module semiconductor device that employs the ceramic material.
Among many insulative ceramic layers, some may have strength that is below design strength. To prevent a dielectric breakdown of module semiconductor devices that are made from such ceramic layers, it is necessary to completely eliminate cracks from the ceramic layers. To achieve this, design stress for the ceramic layers must be set as small as possible. This, however, is impractical to achieve. In this way, ceramic materials have a reliability problem.
An object of the present invention is to provide an insulating substrate having a high breakdown voltage to achieve high reliability.
Another object of the present invention is to provide a method of manufacturing an insulating substrate that has a high breakdown voltage and is reliable.
Still another object of the present invention is to provide a module semiconductor device that has a high breakdown voltage and is reliable.
In order to accomplish the objects, a first aspect of the present invention provides an insulating substrate consisting of insulative ceramic layers, an intermediate layer arranged between adjacent ones of the ceramic layers to join them together, a first conductive layer joined to the top surface of a top one of the ceramic layers, and a second conductive layer joined to the bottom surface of a bottom one of the ceramic layers.
The first aspect joins insulative ceramic layers each having a predetermined breakdown voltage to one another with intermediate layers and arranges a first conductive layer on the top surface of a top one of the ceramic layers and a second conductive layer on the bottom surface of a bottom one of the ceramic layers. The intermediate layers are made of a material that is different from a material of the ceramic layers.
Even if any one of the ceramic layers of the substrate has strength lower than a design value to cause a breakage due to thermal stress, the remaining ceramic layers will be sound to cause no dielectric breakdown.
Generally, an insulating substrate has a creepage surface (to be explained alter) having a low breakdown voltage. Accordingly, it is insufficient for an insulative ceramic layer to have a thickness that secures a required breakdown voltage. Namely, the ceramic layer must have a thickness that secures the required breakdown voltage even at a creepage surface. The present invention employs a plurality of insulative ceramic layers to solve this problem without greatly increasing the thickness of an insulating substrate. Manufacturing thin insulative ceramic layers is more productive and cost saving than manufacturing thick insulative ceramic layers. The thin ceramic layers have a reduced volume, which leads to reduce a probability of defects and improve reliability.
To prevent a breakage of an insulating substrate due to thermal stress, etc., the first aspect selects materials for forming the insulating substrate. These materials will be explained. The insulative ceramic layers of the insulating substrate may be made from a material selected from the group consisting of metal oxides and metal nitrides. The intermediate layers of the insulating substrate may be made of a metal whose yield strength is half or below the fracture strength of the material for the insulative ceramic layers, or metal or ceramics whose thermal expansion coefficient is within a range of xc2x12xc3x9710xe2x88x926/K of that of the material for the insulative ceramic layers. The first and second conductive layers of the insulating substrate may be made of a material selected from the group consisting of copper, aluminum, and alloys of copper and aluminum. If the insulating substrate consists of three or more insulative ceramic layers, the top and bottom ones of the ceramic layers may be made of a material whose strength and fracture toughness are higher than those of a material for the remaining ceramic layers. The insulating substrate may be produced by joining a copper layer to each of the top and bottom surfaces of each insulative ceramic layer and by joining the copper layers together. The insulative ceramic layers, intermediate layers, and first and second conductive layers are joined together by a method selected from the group consisting of soldering method, active metal brazing method, and direct bonding copper method.
To improve the breakdown voltage of the insulating substrate, the first aspect employs special structures. These will be explained. Each end face of each insulative ceramic layer is protruded from the end faces of the first and second conductive layers and intermediate layers by 0.5 mm or more, preferably, 1.0 mm or more. Each corner of the insulative ceramic layers, first and second conductive layers, and intermediate layers may have a radius of curvature of 0.5 mm or larger, preferably, 1.0 mm or larger. Each edge of the insulative ceramic layers may be chamfered by a size of ⅕ or larger of the thickness of the insulative ceramic layer at an angle in the range of 30 to 60 degrees with respect to a vertical. Preferably, it may be chamfered by a size of ⅓ or larger of the thickness of the insulative ceramic layer at an angle of 45 degrees. A creepage surface of the insulating substrate may be provided with an insulator inserted into a gap between the insulative ceramic layers. An end face of the insulator may be protruded from the end faces of the insulative ceramic layers. The surface of each insulative ceramic layer that is exposed to atmosphere may be covered with an insulator that blocks moisture.
Thermal stress acting on each insulative ceramic layer is calculated from statistical data related to the strength of a material of the insulative ceramic layer. If two insulative ceramic layers are laid one upon another to form an insulating substrate, a probability of the two ceramic layers causing a breakage will be one several tens of thousandths. If higher reliability is required, three insulative ceramic layers may be employed to form an insulating substrate to greatly reduce a probability of causing a breakage.
A second aspect of the present invention provides a method of manufacturing an insulating substrate, including the steps of fixing a plurality of insulative ceramic layers at given intervals in a forging die, pouring molten metal into the forging die, forging and solidifying the molten metal to form each intermediate layer between adjacent ones of the ceramic layers to join them together, a first conductive layer on the top surface of a top one of the ceramic layers, and a second conductive layer on the bottom surface of a bottom one of the ceramic layers, and removing excess parts from the solidified metal to complete the insulating substrate.
The xe2x80x9cgiven intervalsxe2x80x9d are set to be proper for forming the intermediate layers when the molten metal solidifies. The step of removing excess parts to complete the insulating substrate may be carried out by machining or electrolytic etching.
The second aspect involves no joint layers formed by soldering method or active metal brazing method, and therefore, causes no strength problem and improves the thermal cycle resistance of the insulating substrate. Compared with the direct bonding copper method, the second aspect involves a large quantity of molten metal when forging the insulating substrate. As a result, the second aspect forms little defects such as voids in each joint interface of the insulating substrate.
A third aspect of the present invention provides a method of manufacturing an insulating substrate, including the steps of joining a copper layer to each of the top and bottom surfaces of each insulative ceramic layer and joining the copper layers together.
The third aspect forms each joint interface of an insulating substrate with the same material, i.e., copper, to prevent a warp and gap from being formed at the joint interface, thereby improving the strength of the joint interface.
A fourth aspect of the present invention provides a module semiconductor device having insulative ceramic layers, an intermediate layer arranged between adjacent ones of the ceramic layers to join them together, a first conductive layer joined to the top surface of a top one of the ceramic layers, a second conductive layer joined to the bottom surface of a bottom one of the ceramic layers, semiconductor chips joined to the top surface of the first conductive layer, and a base joined to the bottom surface of the second conductive layer.
Even if the strength of any one of the insulative ceramic layers that form an insulating substrate is below design strength to cause a breakage due to thermal stress, the remaining ceramic layers of the fourth aspect will be sound to maintain a required breakdown voltage for the insulating substrate. The module semiconductor device of the fourth aspect is capable of continuously operating even if one of the ceramic layers causes a breakage.
According to the fourth aspect, a gap between adjacent ones of the ceramic layers along a creepage surface of the insulating substrate and a gap between the bottom ceramic layer and the base may be filled with insulative sealing resin.